This invention relates to printed wiring boards and more specifically to printed wiring boards using solderless surface mounted devices.
Surface mounted devices (SMDs) are electronic components that are designed to sit on the surface of a printed wiring board (PWB) or another compatible substrate. Components such as resistors, capacitors, diodes, transistors and integrated circuits (IC) may be designed as SMDs and, as such, have either no leads but flat interfacing surfaces or very short leads. The interfacing surfaces or the short leads of these components serve as contact pads which align with corresponding electrical connections on a PWB. For purposes of this discussion, reference will be made only to contact pads on an SMD which could refer to flush interfacing surfaces having no protruding leads or could refer to short leads.
Typically the components are mounted or fabricated within a ceramic or plastic carrier to provide a desired configuration having contact pads on the external surface of the carrier which are electrically connected to the component contained therein.
The advantages of SMDs are numerous. The size of these devices may be 30 to 60% smaller than the traditional leaded components they replace. The holes within the PWBs which accept the leads of leaded components are no longer necessary when utilizing SMDs. For this reason, not only may SMDs be mounted closer together, but SMDs may also be mounted on each side of a PWB. Because of these factors, the overall size and weight of a populated PWB is less than that of a PWB using leaded components.
While surface mount technology has existed for some time, by and large surface mounted devices generally are attached to a PWB using solder. The use of solder to secure SMDs, just as with many other electronic devices, has a number of associated problems. First of all, in order for solder to flow between the SMD and the PWB, the solder must be heated. When the solder is heated, typically due to their proximity the SMD and the PWB are also heated. Occasionally, this high temperature damages the surface mounted device thereby reducing the device reliability.
Secondly, after an SMD is soldered to a PWB, inspection of the newly created solder joint is necessary to verify the integrity of the joint. Depending on the location of the solder joint and the configuration of the SMD, this inspection process, if inspection is even possible, can be difficult and time consuming.
Finally, whenever solder is used, there exists the possibility that the solder may inadvertently be deposited or spattered onto another component or the PWB thereby resulting in contamination to the component or PWB.
Aside from the negative effects associated with the use of solder on SMDs, some SMDs, such as those housing an IC chip, may generate a significant amount of heat that must be dissipated in some manner to avoid damage to the device. Typically, this dissipation is done utilizing a heat sink attached to the opposite side of the PWB from that which the device is attached. Heat dissipation from the device occurs primarily by conductive heat transfer from the device through the PWB and into the heat sink. Given a PWB of a significant thickness, the conductive heat transfer from the device to the heat sink may be so limited that the controlling factor in selection of the chip may be the heat it generates as opposed to operational capability.
Furthermore, the placement of SMDs onto a PWB requires precision such that the contact pads of the SMD are aligned with and contact the associated pads on a PWB. As the number of contact pads on the SMD increases and the pitch between pads decreases, the level of precision with which the SMD must be located upon the PWB also increases to a point where great care must be taken to place the SMD in the proper location on the PWB surface.
It is an object of this invention to provide a PWB assembly whereby SMDs may be mounted and held in contact with a PWB using a device that applies pressure on the SMD forcing it against the PWB such that no solder is used, thereby avoiding the problems of inspection, solder reflow heat and the contamination associated with the use of solder to attach SMDs to PWBs.
It is another object of this invention to provide an assembly whereby the heat transfer from a component may be increased and will not be dependent on the thickness of the PWB to which it is attached.
It is still another object of this invention to provide a PWB assembly which permits placement of an SMD onto a PWB without the necessity of precise initial alignment before the SMD contacts the PWB surface.